This invention relates generally to the amplification of audio signals and more particularly to the amplification of musical instruments.
The desired sound of an instrument amplifier has changed in the past number of years. It has become very desirable to achieve an extremely high gain, overdriven, distorted sound with long sustain, abundant harmonic content and warm, full tonal qualities while avoiding a harsh, raspy characteristic. There are many versions of instrument pre-amplifiers which will produce similar sound qualities. But, when using an amplification factor large enough to produce the desired effect, many noise problems are encountered. Due to the fact that the musical instrument is very low level and generally high impedance, amplified AC line noise (60-50 Hz) and other extraneous signals such as light dimmer interference, radio frequencies and the like become extremely audible. In addition, the musician is faced with the problem of acoustic feedback even at moderate volume levels.
Attempts have been made to solve this problem by the use of noise gates or expander circuits, used either at the input to a high-gain amplifier or pre-amplifier circuit, or after an instrument pre-amplifier. In both cases, the effectiveness is less than desirable and many side effects are encountered.
When a gate or expander is used at the input to a high-gain amplifier, the gate is only effective at removing input noise and will not eliminate the noise of the amplifier circuit itself. When using a gate after a high-gain audio pre-amplifier, other problems are encountered. For example, when the input audio signal is just slightly above the threshold point of operation, or equal to the amplitude of the noise, the noise gate will close and open sporadically when the peak amplitude of the signal exceeds the pre-set threshold point. It is easy to understand that soft passages of guitar music, or virtually any audio source, may have an average signal level equal to the average level of the noise floor. This is where the major flaw of the noise gate is most apparent. Soft passages become nonexistent or pop on and off as the peak amplitude of the input signal exceeds the threshold point.
The use of dynamic noise reduction as described in my U.S. Pat. No. 4,696,044 is a major improvement over the noise gate. But, while circuit noise is reduced, acoustic feedback is somewhat reduced, and the operation of the downward expander virtually eliminates the popping effect encountered with noise gates, other improvements can be made. Line frequency noise and light dimmer interference are still obtrusive when using dynamic noise reduction. It is easy to understand that when an instrument is being played at extremely high levels, saturation of the amplifier is taking place and any audible noise is masked by the audio signal. When the musician plays softly, or stops playing all together, the noise intrusion becomes apparent, especially if the instrument is at full volume. Therefore, the musician must either use a volume control pedal to quickly compensate for the amplification or learn to quickly control the volume control on the instrument itself.